1. Field of the Invention
The present invention relates to an optical multi-demultiplexer used to multiplex or demultiplex wavelength division multiple signals, and more particularly, an optical multi-demultiplexer capable of reducing insertion loss and crosstalk.
2. Description of the Related Art
In the field of optical communication, a wavelength division multiplexing transmission system is examined such that light beams of different wavelengths are loaded individually with a plurality of signals and information capacity is enlarged by transmitting the signals by means of one optical fiber. In this transmission system, optical multi-demultiplexers for multiplexing or demultiplexing light beams of different wavelengths play an important role. Among various other optical multi-demultiplexers, an optical multi-demultiplexer that uses an arrayed-waveguide grating (AWG) holds promise, since it can increase the frequency of multiplexing with short wavelength intervals.
In one such optical multi-demultiplexer used in the wavelength division multiplexing transmission system, it is essential to reduce loss in a wavelength passband in consideration of the wavelength control tolerance of a semiconductor laser beam source, gain characteristics of an optical fiber amplifier, wavelength characteristics of a dispersion compensating fiber, etc. It is also important to ensure sharp rising and falling edges in the passband. Conventionally, there is a proposal to taper an end portion of an arrayed waveguide in order to reduce loss in the wavelength passband. Described in Jpn. Pat. Appln. KOKAI Publication No. 5-313029, for example, is an arrayed waveguide that has a tapered end portion on the interface between an input slab waveguide and the arrayed waveguide.
According to this prior art structure in which the end portion of the arrayed waveguide is tapered, however, a loss is caused by the difference between the respective native modes of the slab waveguide and the arrayed waveguide, so that reduction of loss is limited.
It is found that crosstalk can be reduced by maximizing the width of the slab waveguide and increasing the number of channel waveguides of the arrayed waveguide that are connected to the slab waveguide. If the channel waveguides of the arrayed waveguide are increased in number, however, they are easily influenced by the refractive index distribution and fluctuations of the channel waveguide width. This leads to adverse results including an increase in loss and a worsened crosstalk level.
Crosstalks are calculated in the following manner.
FIG. 22 shows a wavelength characteristic of a channel No. 5 out of eight channels of an AWG of 100 GHz as an example. The criterion for the calculation of crosstalks is not 0 (zero) dB but insertion loss for the center wavelength.
For example, a in FIG. 22 indicates insertion loss for the center wavelength of the channel No. 5. Further, b indicates a crosstalk between channels No. 5 and No. 6; c, crosstalk between channels No. 5 and No. 4; d, crosstalk between channels No. 5 and No. 7; e, crosstalk between channels No. 5 and No. 8; f, crosstalk between channels No. 5 and No. 3; g, crosstalk between channels No. 5 and No. 2; and h, crosstalk between channels No. 5 and No. 1.
The average of all the crosstalks in the channel No. 5 can be given by (b+c+d+e+f+g+h)/7. In this specification, the value calculated in this manner is referred to as crosstalk.
As a power splitter for splitting signal light, on the other hand, a splitter that combines a slab waveguide and channel waveguides is proposed in place of a conventional splitter that is composed of multilayered Y-branches. “An integrated power splitter with ultra-low loss” (Integrated Photonics Research 1999, Santa Barbara, Calif., Jul. 19-21, 1999, pp. 141-143) is reported as an example of the proposed splitter. In this splitter, a semiconductor with a refractive index of 3.0 or more is used as its material, and a high-refraction region with a refractive index higher than that of a core layer is provided in the slab waveguide.
Since this splitter uses a rib waveguide with a core width of 1 μm as its output waveguide, however, its mode diameter is as small as about 1 μm. Since the mode diameter of an ordinary optical fiber ranges from 9 to 10 μm, on the other hand, the mode mismatch (connection loss) in the output waveguide portion is substantial. It is feared that this difference in mode diameter should entail a loss of 13 dB or more. Thus, the loss of the whole splitter, including the splitter's own loss of 6 to 7 dB, inevitably amounts to about 20 dB, a substantial loss.
In addition, the aforesaid high-refraction region in the slab waveguide measures only 2.5 μm by 0.9 μm. Thus, the individual parts have very fine dimensions, and the layer structure is complicated, so that the manufacture of the splitter is subject to variation. Thus, the quality of the splitter lacks stability and reproducibility. It is hard, therefore, to improve insertion loss or the like remarkably by means of a splitter of this type.